Lotta bull about bullatosaurs (was: Conservatism of dinosaur brain cases)

[Dinogeorge@aol.com]

In a message dated 96-05-30 13:49:14 EDT,
jrw6f@faraday.clas.virginia.edu writes:

>Dinogeorge writes:
>
>>Sorry, this is still argument from authority. What >>evidence<< is
>>there that an inflated parasphenoid (wasn't it basisphenoid?)
>>capsule couldn't have occurred more than once within Dinosauria?
>
>Sorry, I have an authority complex :) .
>
> It is referred to in _The Dinosauria_ as Basiphenoid, but Peter
> BucHHoltz told me that Tom HolTz said that the structure on
> ErliC/Kosaurus is the PARAspehnoid (or is homologous
> thereunto)(pers. com. from pers. com.  Objection, hearsay, your
> honor!)

Skull anatomy lesson, braincase section:

PARASPHENOID: The bone at the very front of the braincase. In certain
theropods (e.g., tyrannosaurids, ornithomimids, troodontids) it comes
to a rather sharp point, visible in lateral view of the skull when you
look right through the orbits. The point extends into the nasal cavity
and may have something to do with turbinals, an issue currently being
debated; but in some theropods it may lie lower and articulate to some
degree with the rear palatal bones. Some references call the point the
>>cultriform process<< of the parasphenoid.

BASISPHENOID: The bone at the base of the braincase. It is the most
prominent element of the floor of the skull behind the palate and
articulates with the pterygoids on either side via two sometimes quite
elongate >>basipterygoid processes<<. It may fuse with the
parasphenoid as one goes up the front of the braincase, between the
pterygoids, from below. At the back of the skull it contacts (and
often fuses with) the basioccipital, which among other things forms
most if not all of the occipital condyle. Two big lumps, the
>>basisphenoid tubera<< also extend downward, backward, and outward
(though not always in all three directions) from the bottom of the
basisphenoid. In saurischians (that is, non-ornithischian dinosaurs),
the basisphenoid is often strongly pneumatized.

The important thing is that these are two different skull bones.

In Ornithomimidae, the PARAsphenoid is greatly swollen behind the
cultriform process, and hollow with thin walls. This swelling is
clearly visible through the huge orbits. In Troodontidae, this element
is also enlarged, but not to the extent seen in Ornithomimidae. (In
the purported Triassic ornithomimosaur _Shuvosaurus_, the parasphenoid
is narrow and unswollen--so IF _Shuvosaurus_ is an ornithomimosaur,
this is either a major reversal or the parasphenoid capsule developed
separately in Troodontidae and Ornithomimidae. The other alternative,
that _Shuvosaurus_ is not an ornithomimosaur, is more likely.)  The
swollen >>parasphenoid<< capsule is the key character diagnosing
Bullatosauria.

In _Erlikosaurus_, the parasphenoid does NOT have a swollen capsule,
and it is NOT clearly visible through the orbits as in ornithomimids
and troodontids. In the description of the _Erlicosaurus_ [sic] skull
by Clark et al., the parasphenoid is not described separately, and
somehow the cultriform process is depicted as extending from the
>>basisphenoid<< in between the pterygoids and vomers. In the text,
however, the authors describe this region as the parasphenoid and
parabasisphenoid. (Perhaps this is partly responsible for the
confusion in the opening sentences above?) Indeed, the entire
palatal-braincase region of _Erlikosaurus_ is topologically quite
different from that of ornithomimids and troodontids.

On the other hand, in _Erlikosaurus_ the BASISPHENOID is very highly
pneumatized, with lots of pneumatic pockets and very thin walls. It is
strongly "inflated" relative to the condition in ornithomimids and
troodontids, ruthlessly shoving the pterygoids aside. It is quite
differently shaped from the corresponding bone in
bullatosaurs. Instead of being at the front of the braincase (where
the parasphenoid is), the swollen part is positioned more or less
below the occipital condyle at the back of the skull.  The
configuration of the basisphenoid and surrounding bones of
_Erlikosaurus_, not to my surprise, bears a fairly strong resemblance
to the corresponding part of the skull of _Plateosaurus_ and primitive
ornithischians (e.g., stegosaurs).

Clark et al write: "Troodontidae and Ornithomimosauria both possess an
enlargement of the parabasisphenoid [sic] that presumably contained
pneumatic spaces associated with the eustachian tubes. Although _the
parasphenoid region is not enlarged_ [italics mine] in _Erlicosaurus_
[sic], the basisphenoid is distinctly enlarged (figs. 6, 8), as in
these taxa but unlike other nonavialian theropods." Well, the
BASIsphenoid is NOT "distinctly enlarged" in any bullatosaur (check
the palatal view of the skull of _Gallimimus_ on p. 228 of _The
Dinosauria_, for example) in which the complete element has been
illustrated. And the eustachian tubes (in the sides of the braincase)
have nothing to do with the parasphenoid (at the front of the
skull). The swollen parasphenoid capsule and swollen basisphenoid are
two quite different features of the skull of bullatosaurs and
_Erlikosaurus_. The presence of the swollen parasphenoid capsule in
bullatosaurs but not in _Erlikosaurus_, and the swollen basisphenoid
in _Erlikosaurus_ but not in bullatosaurs, supports a profound
taxonomic separation of _Erlikosaurus_ from bullatosaurs.